xfs: split out a new set of read-only iomap ops

Start untangling xfs_file_iomap_begin by splitting out the read-only
case into its own set of iomap_ops with a very simply iomap_begin
helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>